The production of electricity through the conversion of energy from underground geothermal reservoirs is a growing industry. Unfortunately, large scale production of electricity from geothermal sources produces considerable amounts of waste in the form of residual brine sludges. These sludges contain varying concentrations of toxic metals which require expensive disposal at hazardous waste disposal sites.
Hydrothermal systems utilize subsurface reservoirs of either steam (vapor-dominated) or hot water (liquid-dominated). Water in these systems is usually derived from surface water that percolates downward through sediments or fissures to a heat source such as hot dry rock or molten magma. The heated water then rises toward the surface in the form of geysers or hot springs. If the pressure on the water within the reservoir is insufficient to prevent boiling, a vapor phase consisting of dry or super-heated steam will form. Such a system is readily exploitable for generating electricity.
When the pressure exceeds the vapor pressure of the brine in the reservoir there is little vapor phase, resulting in a liquid-dominated hydrothermal system. Liquid-dominated hydrothermal systems are more difficult to utilize directly since they contain dissolved mineral salts which pose serious scaling and corrosion problems in production equipment and injection wells.
The brine from a liquid-dominated system is usually flashed (i.e., pressure is released resulting in an instantaneous expansion to a vapor phase) to a vapor which drives a turbine. The residual liquid brine from the flash may contain precipitated solids that render the residual liquid brine inappropriate for reinjection into a reservoir (due to the potential for injection well-clogging). The precipitated waste solids may be removed by a clarifier and the brine may be filtered. After filtering, the brine can be reinjected.
Geothermal power plants also generate waste during well drilling and plant operations. Well drilling waste consists of drilling muds, brines, and residues and operational waste consists of cooling tower and separator blown down sludges. These slurries and brines also pose serious problems for reinjection. Additionally, many of these precipitated solids are toxic.
The chemical composition of geothermal brine fluids varies greatly, is source specific, and can vary over time. Geothermal brine fluids may contain a number of different dissolved metals such as chromium, vanadium, titanium, antimony, nickel, bismuth, tin, silver, cadmium, beryllium, selenium and others. These solids precipitate out during the processing of the superheated fluids, are highly concentrated and enriched in a variety of salts.
Safe waste disposal is of increasing concern worldwide. In California for example, all solid waste produced in the Imperial Valley must be analyzed for regulated substances using the California Department of Health Services (DOHS) standard analytical techniques. "Identification and Listing of Hazardous Waste Under RCRA (Resource Conservation and Recovery Act)", Subtitle C, Section 3001, EP Toxicity Characteristics, USEPA. (May 1980). A solid waste is deemed hazardous if it contains a regulated substance (e.g., Zn, Cr, Pb, As, Cu) at a level exceeding the total threshold limit concentration (TTLC). Waste is also considered hazardous if the leachable level of any regulated substance is above the DOHS soluble threshold limit concentration (STLC). The California Department of Health Services (DOHS) Soluble and Total Threshold Limit Concentrations (STLC and TTLC) calculated by standard analytic techniques (EPA, 1980) are shown in Table 1.
TABLE 1 ______________________________________ The California DOHS Soluble and Total Threshold Limit Concentration Values of Inorganic Toxic Substances in Hazardous Waste STLC TTLC Inorganic Substances (mg/1) (mg/kg) ______________________________________ Antimony and/or antimony compounds 15 500 Arsenic and/or arsenic compounds 5.0 500 Barium and/or barium compounds 100 10,000* (excluding barite) Beryllium and/or beryllium compounds 0.75 75 Cadmium and/or cadmium compounds 1.0 100 Chromium (VI) compounds 5 500 Chromium and/or chromium 560 2,500 (III) compounds Cobalt and/or cobalt compounds 80 8,000 Copper and/or copper compounds 25 2,500 Flouride salts 180 18,000 Lead and/or lead compounds 5.0 1,000 Mercury and/or mercury compounds .2 20 Molybdenum and/or molybdenum 350 3,500 compounds Nickel and/or nickel compounds 20 2,000 Selenium and/or selenium compounds 1.0 100 Silver and/or silver compounds 5 500 Thallium and/or thallium compounds 7.0 700 Vanadium and/or vanadium compounds 24 2,400 Zinc and/or zinc compounds 250 5,000 ______________________________________ * excluding barium sulfate Note: STLC and TTLC values are based on the concentrations of the elements, not the compounds. TTLC values are calculated on a wetweight basis. The limits of elemental metals apply only if the substances are in a friable, powdered, or finely divided state.
Due to the high cost of hazardous solid-waste disposal and the long-term liability, the continued use of hypersaline geothermal brines in the production of electricity hinges upon the reduction or elimination of toxic solid-waste generation.
Microbial leaching (bioleaching) of low-grade ores using bacteria of the genus Thiobacillus (T.) has been utilized by the mining industry for the recovery of copper and uranium. Microbial leaching involves the dissolution of soluble materials catalyzed by microorganisms. Thiobacilli oxidize certain metallic sulfides into water-soluble sulfates by acting as catalysts in the production of sulfuric acid from elemental sulfur (LeRoux, N. W., New Scientist, pp. 12-14, (September 1969)).
Most Thiobacilli are mesophilic, i.e., they thrive at moderate temperatures. Both T. ferrooxidans and T. thiooxidans are chemolithotrophic or "rock-eating", i.e., they obtain their energy from the oxidation of inorganic compounds found in rock. Both species of bacteria obtain their carbon requirements from carbon dioxide in the air, i.e., they are autotrophic (Brierly, C. L., Science 78:44-53 (1978)).
Thiobacilli ferrooxidans and Thiobacilli thiooxidans bioleach metals into solution via different mechanisms. T. ferrooxidans solubilizes metals through a series of oxidation-reduction reactions to form soluble metal sulfates (Kelly, O. P. and C. A. Jones, Metallurgical Applications of Bacterial Leaching and Related Microbiological Phenomena, Academic Press, New York, pp. 19-43 (1978)).
T. ferrooxidans are gram-negative nonsporing rod-like organisms measuring 0.5-0.7 .mu.m, which move about using a single polar flagella. Their energy requirements are met by the oxidation of inorganic sulfur, sulfide compounds, and iron compounds (Silver, M., Metallurgical Applications of Bacterial leaching and Related Microbiological Phenomena, Academic Press, New York, pp. 19-43 (1978)). T. ferrooxidans solubilizes certain metal ions through the following oxidation-reduction reactions (Bosecker, K and Kursten, M., Process Biochemistry, pp. 2-4 (October 1978)). EQU 2FeS.sub.2 +7O.sub.2 +2H.sub.2 O.fwdarw.2FeSO.sub.4 ( 1) EQU 4FeSO.sub.4 +O.sub.2 +2H.sub.2 SO.sub.4 .fwdarw.2Fe.sub.2 (SO.sub.4).sub.3 +2H.sub.2 O (2) EQU MS+Fe.sub.2 (SO.sub.4).sub.3 .fwdarw.MSO.sub.4 +2FeSO.sub.4 +S.degree.(3) EQU 2S.degree.+3O.sub.2 +2H.sub.2 O.fwdarw.2H.sub.2 SO.sub.4 ( 4)
where M is the metal of interest. Reactions (1), (2), and (4) are catalyzed by T. ferrooxidans. Reaction (3) proceeds in the absence of bacteria. Ferric ions produced from the oxidation of ferrous ions in reaction (2) are reduced in reaction (3). The insoluble metal sulfides are oxidized in reaction (3) to produce metal sulfates and elemental sulfur. The elemental sulfur produced in reaction (3) is then oxidized in reaction (4) to form sulfuric acid. In the absence of iron, the acid produced can oxidize metal sulfides. Leaching is accomplished because the sulfates produced are soluble.
T. thiooxidans are gram negative, rod shaped bacteria 1.0-2.0 .mu.m long and 0.5 .mu.m across. Like T. ferrooxidans, they also move about by means of a single, polar flagellum. T. thiooxidans also oxidize elemental sulfur and thiosulfate, but are incapable of oxidizing iron (Cripps, R. E., Extern, 9(40):200-216 (1980)). These bacteria catalyze the formation of sulfuric acid via reaction (4). The sulfuric acid thus generated solubilizes metal sulfides by the following reaction: EQU 2MS+2H.sub.2 SO.sub.4 +O.sub.2 .fwdarw.2MSO.sub.4 +2H.sub.2 O+2S.degree.(5)
Reactions (4) and (5) constitute the Thiobacillus catalyzed acid leaching cycle.
It is known that mixed cultures of T. ferrooxidans and T. thiooxidans are more effective than either organism alone in the leaching of certain ores. This is due to the complementary nature of reactions (2) to (4). The elemental sulfur generated in reaction (3) is oxidized by both T. thiooxidans and T. ferrooxidans in reaction (4) to produce sulfuric acid. However, none of the above-references teach or suggest the treatment of geothermal waste.
It has been previously shown by the subject inventors that acidophilic microorganisms can be used as the "active agents" in the detoxification of geothermal brine residues. A preliminary design for a process has been suggested and a technical and feasibility study of this process has been earlier described by the subject inventors (Premuzic, et al., Geothermal Resources Council, TRANSACTIONS, Vol. 12 (October 1988)). The efficiency of metal sulfide solubilization by several strains of Thiobacillus thiooxidans and Thiobacillus ferrooxidans was also studied.
Acidophilic bacteria have been used to leach sulfide minerals such as zinc and copper. For example, U.S. Pat. No. 2,829,964 to Zimmerley, et al. describes a cyclic process involving the use of a ferric sulfate sulfuric acid solution carrying cultures of iron oxidizing autotrophic bacteria. The bacteria involved appear akin to Thiobacillus ferrooxidans. The bacteria were bred in successively greater concentrations of dissolved metals, resulting in strains of bacteria which tolerate relatively high metal concentrations. The bacterial leaching process may be enhanced by the addition of certain nutrients to the leaching solution. Zimmerley, et al. do not however teach or suggest the treatment of waste and do not use mixed strains of Thiobacillus.
U.S. Pat. No. 3,433,629 to Imai, et al. describes the use of Thiobacillus thiooxidans for dissolving and recovering manganese in the form of water soluble salts from manganese ores. The use of mixed cultures of Thiobacillus is not taught or suggested and the patent does not address geothermal waste treatment.
U.S. Pat. No. 4,033,763 to Markels describes the recovery of metals from waste waters by bacterial action. There is very little discussion as to what type of bacteria is used and Thiobacillus ferrooxidans and Thiobacillus thiooxidans are not mentioned. Markels uses "sewage-type" bacteria to imbibe metals in a sludge (i.e., the metals are not dissolved).
U.S. Pat. No. 4,530,763 to Clyde discloses the use of Thiobacillus ferrooxidans to remove palladium from waste fluid. Thiobacillus ferrooxidans is attached to a porous fiber webbing. Palladium then attaches (it is not dissolved) to the Thiobacillus ferrooxidans when the webbing is contacted with waste fluid. After a sufficient period of time, the webbing is removed from the fluid and palladium is separated from the webbing.
U.S. Pat. No. 4,664,804 to Morper, et al. describes a process for the removal of heavy metals from waste water. In this process, anaerobic sludge is added to the waste water. Contrary to solubilization, heavy metals are absorbed (not dissolved) by the anaerobic sludge and are thereafter separated from the sludge.
U.S. Pat. No. 4,725,357 to Downing, et al. describes a process for the removal of dissolved selenium from water by treatment in a reactor containing a microbial Bacillus biomass. Downing, et al. do not address metal sulfide dissolution by microbial action.
U.S. Pat. No. 4,732,608 to Emmett, Jr., et al. describes a method and apparatus for use in a bioleaching process for metal bearing solids (such as a mineral ore). The method encompasses the placement into a tank of a grounded metal bearing solid, water, oxygen, carbon dioxide, nutrients and a species of microorganism capable of oxidizing some portion of the metal bearing solid and obtaining energy from that oxidation. Emmett, Jr., et al. disclose the use of Thiobacillus ferrooxidans and Thiobacillus thiooxidans in leaching treatments for the solubilization of copper from low grade ores. The Emmett, Jr., et al. apparatus includes a plurality of bioreactor vessels with a means for introducing air bubbles into the slurry and a plurality of radial arms which rotate and agitate the slurry. The process and apparatus are specifically directed for use in processing precious metal-bearing pyrites. The process and apparatus may be used for leaching gold, silver and platinum from sulfide containing ore solids, but are not used in the treatment of geothermal waste.
U.S. Pat. Nos. 4,740,243 to Krebs-Yuill, et al. and 4,752,243 to Wu, et al. describe processes for recovering metal from a metal sulfide containing ore with Thiobacillus ferrooxidans. This involves contacting an ore with an aqueous acidic composition and at least one reducible manganese-containing material in the presence of Thiobacillus ferrooxidans. By normal acclimation techniques, the manganese tolerance of the bacteria is increased to greater than about 4 weight per cent. Neither of these disclosures relate to the dissolution of metals in a geothermal sludge, but rather describe leaching metals (such as gold and silver) from a metal sulfide containing ore.
No prior art describes a process for the dissolution of metal sulfides in a geothermal sludge involving the use of Thiobacillus ferrooxidans. Nor does any describe an apparatus for use in the bioleaching of metals from a geothermal sludge.
An object of the present invention is to remove precipitated solids from a geothermal brine or sludge through bioleaching. Since the subject processes are readily (and economically) suitable for commercial application, the subject invention presents a solution to a long felt need in the art.
A second object of the present invention is to provide a biological treatment at high temperatures, 50.degree.-60.degree. C. or even higher, for geothermal brine or sludge which renders the geothermal sludge non-toxic and lowers the cost of waste disposal.
By employing the methods and bacteria of the subject invention, it is possible to biologically reduce the toxic metal content by reducing the concentration of metal sulfides and other metal salts in a geothermal sludge or brine. In addition, this treatment reduces the toxicity of the sludge or brine and minimizes the cost of toxic waste removal by decreasing the amount and volume of toxic waste produced.
Further, by reducing the amount of solid metal salts, the present invention diminishes the scaling and clogging of pipes or other conduits used to transport brine or sludge.
The subject low-cost processes for the removal of toxic metals from and volume reduction of geothermal residues include:
1. efficient removal of metals by means of biochemical processes;
2. the flexibility to be operated at different temperatures and sludge concentrations;
3. identification of key process variables; and
4. cost-efficiency in the detoxification process.